50% of surgical patients with acquired esotropia achieve good eye alignment with one surgery. New evidence indicates that with the preoperative use of prisms, good eye alignment can be achieved with one operation in approximately 85% of surgical patients. A multicenter randomized, prospective, clinical trial with careful attention to standardization and unbiased outcome evaluation is proposed to determine: 1. The overall beneficial effect of the prism adaptation procedure by comparing (a) prism adaptation responders who have surgery for the prism adapted angle, and prism adaptation non-responders who have surgery for the original angle of deviation to (b) patients who do not have prism adaptation and have surgery for the original angle of deviation; 2. Whether patients with acquired esotropia who respond to prism adaptation are more accurately corrected by operating for the prism adapted angle or the original angle of deviation; 3. Whether patients who respond to prism adaptation by developing a new stable angle of deviation have a better surgical result than patients who do not respond to prism adaptation; and 4. Whether certain input variables are useful in predicting which patients are more likely to benefit from prism adaptation. To answer these questions a double randomization study has been designed. Three-fifths of the patients will be selected to go through prism adaptation prior to surgery. Two fifths of the patients will have surgery based on the amount of crossing measured using routine examination techniques. Of those patients who respond to the prisms, one-half will have surgery based on the amount of prism required to stabilize the deviation while the rest will have surgery based on the amount of crossing originally measured. If prism adaptation leads to more successful surgery, its adoption can have a significant effect in decreasing the cost and morbidity for treatment of acquired esotropia. Approximately 2,500 reoperations for acquired esotropia are performed each year in the U.S. If these are reduced by two-thirds through utilization of prism adaptation, the saving would be between $3.2 million and $4.8 million per year. Morbidity would be reduced with fewer operative complications occurring, less time lost from school or work, and less psychological trauma to the patient.